An Apparatus for papermaking generally includes three sections, formation, pressing and drying. Papermakers fabrics form and transport an aqueous paper web through the papermaking apparatus.
A forming fabric generally, consists of metallic wire and/or synthetic material such as nylon or polyester. In the formation of some paper grades, the water slurry may be heated to improve the drainage, formation or other desirable characteristics. As the forming fabric travels from the head box to couch in a papermaking machine, water is removed and both the sheet being formed and the forming fabric tend to cool in temperature. Further cooling of the forming fabric occurs in the return section. The addition of showers, either hot or cold, also influences temperature variations of the forming fabric. The abrupt change in temperature has been known to cause dimensional change in the length or width of the forming fabric which can, depending upon the material and construction used, be either a growth or shrinkage as the temperature changes. The change in the fabric dimension is typically very rapid and as a result, ridging, wrinkling, guiding and take-up problems can arise.
In the pressing section of a papermaking machine, the variation in temperature tends to be less drastic, however hot or cold showers used for cleaning the fabric or felt can cause rapid changes in the temperature of the felt. The change in temperature can cause the felt to wrinkle, guide poorly or cause a change in the porosity or permeability of the felt.
The drying section of a papermaking machine may consist of from one to as many as six sections with both top and bottom felt positions. Currently, some dryer felts have been installed in which the felt runs alternately on both the top and bottom positions. Drying is generally accomplished by heated drying cans which are from 4 to 6 feet in diameter. Alternatively, the sheet may be dried using a thru dryer, radiant heat and/or radio frequency.
Variations in temperature along the fabric in the machine direction or across the fabric in the cross direction can be considerable, both between various paper machines and within a given paper machine. The dryer fabric tends to increase in temperature as the fabric proceeds through the machine. The temperature across the dryer fabric in the cross-machine direction also tends to vary. The drive side of the paper machine or the back tends to restrict air flow because of the presence of gears, piping, etc. Whereas the front of the papermaking machine often is more open and permits air to flow freely. This differential between front and back tends to create a non-uniform temperature profile across the fabric. Also, when pocket ventilation is not uniform, moisture laden air is not removed and the moisture profile will vary in the cross-machine direction. The variations in moisture will cause differences in the temperature profile of both the dryer fabric and the paper sheet being produced Placement and operation of dryer can siphons and dryer can flanges are known to cause temperature differences.
Some dryer fabrics are woven as endless belts where the filling yarns serve as the machine direction yarn and the warp yarn as the cross-machine direction yarn. Most dryer fabrics are, however, woven as a flat belt in which the warp is the machine direction yarn and the filling is the cross-machine direction yarn. In such fabrics, it is common to form an endless fabric loop incorporating a clipper seam, pin seam or other joining means.
Some papermakers fabrics are non-woven. Fabrics have been used in papermaking which are comprised of helical spirals wherein the spirals are intermeshed and serially connected by pintles to form an endless belt, for example, see U.S. Pat. Nos. 4,528,236, 4,567,077 and 4,654,122.
In the past, many paper mills have experienced certain problems with papermakers fabrics during the papermaking operation. Some of the reported problems include snaking, guiding, bowing, yo-yo and instability such as distortion, wrinkling, slack middle, roping-up and slack edges.
Snaking is characterized by an oscillation or whipping action of the dryer fabric as it runs on the machine. Sometimes the side to side movement is inherent in the dryer fabric and occurs once for every revolution and at the exact same location of the fabric. Snaking may be caused by improper dryer fabric manufacture, poor installation technique, improper operating procedures and faulty equipment.
Guiding is the steering of the fabric so that it stays on the machine with only periodic and slight movement of the fabric side to side. Guiding is controlled by a mechanical guide paddle, air, light or other sensing device that detects movement of the fabric and then causes the movement of a guide roll to continuously maintain the proper position of the fabric on the machine.
Bowing is associated with the center of the fabric being offset either in a leading or trailing manner as the fabric runs on the machine.
The term "yo-yo" is associated with the fabric changing excessively in length from a sheet-on to a sheet-off condition. To counteract this movement, the take-up roll will move to maintain constant tension of the fabric.
Distortion usually is associated with small areas of the fabric being out of shape, cocked or otherwise misaligned.
The term "wrinkling", applies to creases, ridges or folds in the fabric and may either be straight in the machine direction of the fabric or occur diagonally across the fabric.
The term "slack middle", refers to when the fabric is slack or baggy in the running center of the fabric.
Roping-up is a term used when the fabric runs off the machine and gathers together in a narrow mass or band while it is still running.
The term "slack edge" is used when either the running back or front edge of the fabric is loose, droops or forms a continuous bulge while the remainder of the fabric is running flat or smooth.
The cause of many of these problems in the past was not clearly understood and only occasionally could one relate a particular fabric problem to a machine fault, failure of a guide roll mechanism, machine roll misalignment or other known fault. While all of these problems are a nuisance, consistent and proper fabric manufacturing methods tend to minimize many of the problems encountered.
One of the most serious problems with respect to woven fabrics is slack edges. Even when manufacturing conditions for the fabric are carefully controlled, the problem of slack edges will occur. The problem of slack edges shows itself when the center of the fabric is flat for its entire running length and the running edge or edges tend to bulge or droop. On some designs, the fabric may tend to be slack in the middle rather than on the edge, but this is an exception rather than the general rule. If edge slackness is excessive, the guide paddle will not operate properly and the fabric will run off the machine, causing possible damage to the fabric or even the paper machine itself. In the dryer section, the paper sheet may not be held in intimate contact with the dryer can and sheet cockle on the edge or other problems may occur. All of the problems cited tend to reduce running efficiency and increase costs.
A review of field performance data of woven fabrics has indicated that slack edges occur on the fabric front edge ten times more often than on the back edge of the fabric. Often when a machine is fully hooded, slack edges may only appear when the hood is raised, but disappear when the hood is lowered. Dryer can flanges are also known items that cause dryer can and fabric temperature differences. It was discovered that the front edge is more slack edge prone because the front edge of the machine is open and thus more subject to air drafts and temperature fluctuation. In the case of a hooded machine, when the hood is closed, the fabric tends to reach both moisture and temperature equilibrium and therefore, difficulties in fabric slackness occur less often.
A further study revealed that certain paper machines are more prone to have slack edges than others. Often, when the thick, closed, older, low permeability felts were run, they performed very well, however, when the newer high permeability open mesh fabrics are used, the fabric may have slack edges.
With respect to spiral fabrics, fabric failure due to lack of dimensional stability is much more frequent. Not only is there a relatively high rate of slack edge and slack middle problems, but spiral fabrics have demonstrated frequent problems with guiding, yo-yoing, snaking and oscillation.